Electrical friction sleeve cone penetrometer

ABSTRACT

A quasi-static electrical friction sleeve cone penetrometer probe which includes a main body, portion of the body being a cone load cell connectible to a cone for measurement of cone resistance. A friction sleeve load cell is connectible to the main body, and a friction sleeve is connectible to the friction sleeve load cell for separate measurement of side friction. Each load cell has strain gauges attached around an outer surface, and balancing circuitry located within the cell. The friction sleeve is supported against lateral forces by three annular sleeves located on the main body, friction sleeve load cell and the cone.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application is a continuation-in-part of application Ser. No.092,263 filed Nov. 8, 1979, now abandoned.

A penetrometer is a device for measuring the penetrability ofsemisolids. The first penetrometer was produced by Collin in France in1846, and for more than 100 years penetrometers were mechanical devicesof increasing sophistication which included a probe portion which waspushed or driven into the ground, which produced by mechanical means anindication of the resistance of the soil to the probe.

There have developed two basic types of penetrometers, dynamic andstatic (actually quasi-static). This invention is concerned withquasi-static penetrometers, which are advanced into the ground at asubstantially constant velocity, and produce cone resistance readings atvarious depths, and in particular with electric quasi-static frictionsleeve cone penetrometers which have a cone for measurement of coneresistance, and a friction sleeve for measurement of side friction, andin which the measured quantities are transmitted electrically from thoseelements and displayed on electrically operated equipment.

Present penetrometers are most commonly used to define the natural soilprofile or stratigraphy at a site. A knowledge of the geology of anarea, combined with penetrometers results, enables the soil profileacross the site to be determined. The description of each layer isobtained from characteristic patterns of the cone resistance (R), sidefriction (F), the friction ratio (F/R).

This information is an essential prerequisite to any major buildingconstruction or civil engineering project at a particular site, the onlyalternatives, in situ load tests on full scale foundations, laboratorytest on indisturbed soil samples and in situ testing of soils, beingslow and expensive.

2. Description of the Prior Art

The only practical prior quasi-static friction sleeve cone penetrometeris the Fugro, which dates from 1970, and is described in ThePenetrometer and Soil Exploration by G. Sanglerat, Elsevier, 1972.

The Fugro consists of a probe which is connectable to pushing rods whichare used to advance the probe into the ground from a drilling rig,usually mounted with the ancillary equipment for recording anddisplaying information from the probe, in a heavy motorized vehicle.

The probe consists of an adaptor for connection to the pushing rods, anda body which contains two load cell portions, one of which is connectedto the cone, and the other of which is connected to the friction sleeve.Strain gauges are provided on each load cell, and wiring connected tothe strain gauges is run through a cable to the recording and displayingequipment.

It should be appreciated that F is much smaller than R, in the vicinityof 10⁻³ R, and accordingly it is essential that the smaller quantity beaccurately measured. In the Fugro, F is determined by measuring F+R, andthen by measuring R, and subtracting R from F+R to find F. As R and F+Rare relatively large quantities, the accuracy of the value of F producedby subtraction of one of those quantities from the other is not great.

In addition, the Fugro suffers from structural weaknesses in the bodyand the friction sleeve, the latter being connected to the body at onlyone point, and being subject to lateral forces from the soil and rocksand the like encountered during advancement of the probe. The Fugro alsouses mains power operated equipment in the large vehicle, which makesthe setting up of a Furgo at a site complex and costly exercise.

BRIEF SUMMARY OF THE INVENTION

The invention provides, in a quasi-static electrical friction sleevecone penetrometer, a penetrometer probe including a body, portion ofwhich is a first load cell, and a second load cell removably attached tosaid body, a cone removably attachable one of said load cells, and afriction sleeve removably attachable to the other of said load cells,each of said load cells having transducers and balancing circuitryassociated therewith.

It is an object of this invention to provide an improved quasi-staticelectrical friction sleeve cone penetrometer which produces a continuousrecord of cone resistance and side friction.

It is another object of this invention to provide a quasi-staticfriction sleeve cone penetrometer having a friction sleeve supported forresistance to lateral forces.

It is a further object of this invention to provide a quasi-staticfriction sleeve cone penetrometer which produces cone resistance, sidefriction, and friction ratio measurements of such accuracy that it maybe used in the analysis of any resistive material.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a sectioned elevation of an assembled penetrometer probe, withthe strain gauge and wiring details omitted;

FIG. 2 is a partly-sectioned perspective view of the friction sleeve ofthe probe;

FIG. 3 is a partly-sectioned perspective view of the main body and coneload cell of the probe;

FIG. 4 is a partly-sectioned perspective view of the friction sleeveload cell of the probe; and

FIG. 5 is a block diagram of circuitry of the penetrometer, locatedwithin the probe and externally of the probe.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This embodiment of the invention is described in relation to the primaryapplication of penetrometers, soil analysis and exploration. However, itmust be stressed that the penetrometer of this embodiment, as a resultof the accuracy of its results, may be used in the analysis of anyresistive medium, such as cheese, soil conservation, detection ofminerals in beach sands, forestry, ship design, archaeology, viscousfluid studies, aeronautical studies and agriculture generally.

Referring firstly to FIG. 1, there is shown a sectioned assembledpenetrometer probe according to this invention. The probe consists of amain body and cone load cell 10 (shown in more detail in FIG. 3), towhich is secured an adaptor 12, the elements 10 and 12 being screwedtogether by means of co-operating screwthreads 32. The adaptor 12 isprovided with a tapering internally screwthreaded portion 36 into whichthe lowermost rod of a series of pushing rods (not shown, but referredto later) is screwed.

Towards the `adaptor` end of body 10, there are provided annular grooves20, 24, and a recessed annular shoulder 28. In these grooves there areseated respectively, O-rings 22 and 26, and an annular sleeve 30.Located next to the shoulder 28, is an externally screwthreaded portion64 (FIG. 3) of a diameter less than that of the shoulder. The remote endof the body is a cone load cell 11, and consists of a wide annularrecess 54, and an externally threaded portion 52, to which the cone 16may be screw-fitted.

A friction sleeve load cell 14 (FIG. 4) is screwed to body 10, by theco-operation of load cell internal screwthread 80, and external bodyscrewthread 64. The cell 14 is securely attached to body 10 by a grubscrew located in an internally screw threaded aperture 78. Load cell 14has a wide annular recess 71, and an externally screwthreaded portion 70at the end opposite to that which is screw-fitted to body 10. An annularrecessed shoulder 66 is provided at this end of the cell 14. A thinsleeve 68 of polytetrafluoroethylene is provided between portions 70 and71 of the load cell 14 and the main body 10.

A friction sleeve 18 is secured to load cell 14 by the co-operation ofthe sleeve internally threaded portion 82 and load cell externallythreaded portion 70. Finally, to complete the assembly of elements inthe penetrometer probe, the cone 16 is screwed onto externally threadedportion 52 of body 10.

Cone 16 has annular grooves 38, 42, and a recessed annular shoulder 46,the reverse of grooves and shoulder 20, 24 and 28. Seated in thegrooves, respectively, are O-rings 40 and 44, and seated on the shoulder46 is an annular sleeve 48.

It can be seen that when friction sleeve 18 is screwed onto load cell14, the inner end rides over sleeve 30 and O-ring 26, and abuts O-ring22. Similarly, when cone 16 is screwed onto body 10, sleeve 48 andO-ring 44 ride under friction sleeve 18, and O-ring 40 abuts the outerend of sleeve 18.

Preferably, the O-rings are formed from a tough elastomeric material,and the sleeves 48, 68 and 30 are all formed frompolytetraflouroethylene. The sleeves may be glued into place with anepoxy resin or glue. The first material provides for the absorption ofshocks and limited movement, and the second material, as used in arelatively thin annular sleeve, is extremely strong, and the threesleeves thus resist lateral forces on the friction sleeve 18, which candistort sleeve friction readings. The use of thin sleeves alsoeliminates the need for deep grooves to seat thicker sleeves, and thusthe elements of the probe are strengthened.

Apart from the O-rings and sleeves, and the strain gauges, printedcircuits, balancing bridge, and cabling (to be described hereinafter)the penetrometer probe elements are preferably made from a strong, hardsteel. The friction sleeve 18 is preferably of tempered steel having ahardness of between 45 and 48 Rockwell C. The cone 16 is preferablymanganese tool steel, oil quenched and tempered. The strength of thecone and sleeve is an important factor, as the cone and sleeve containthe material-contracting surfaces, and weakness therein can lead tofaulty results.

The cone 16 has a standard cone angle of 60°, and a conical surface areaof 10 square centimeters. The cone also has a shoulder portion 17, whichallows some wear of the cone surface, without allowing distortion of thefriction sleeve. The friction sleeve has a cylindrical surface of 150square centimeters. The penetrometer probe construction of FIGS. 1 to 4enables the probe to be resistant to lateral forces on the sleeve,impacts on the cone, and bending of the main body of the probe.

The load cells 11 and 14, being connected to cone 16 and friction sleeve18 respectively, enable simultaneous and distinct measurement of coneresistance and side friction to take place. In order to quantify thesemeasurements, transducers in the form of strain gauges and affixed toeach load cell.

On load cell 11, six double strain gauges 58 are secured to surface 54by an expoxy glue or the like. The strain gauges, each of which hasportions to measure axial and tangential stresses, are arrangedsymmetrically around the surface. A flexible printed circuit strip 56 isalso secured to and around surface 54. The strip has staggeredconductors in two rows parallel to the longitudinal axis of the strip.The gauges are electrically connected to the printed circuit, and fromthere the wiring is taken through an aperture 60 to the hollow centralsection 62 of body 10 and cell 11. In this hollow section, balancingbridge and associated circuitry (not shown in FIG. 3, but to bedescribed hereinafter) is located, so that the temperature of the cellaffects the strain gauges and the balancing circuitry. From thecircuitry, the wiring extends through the hollow centre 62 into a cable(not shown) which extends through adaptor 12 and the pushing rods to thedisplay and ancillary equipment, to be described hereinafter.

Load cell 14 has strain gauges 72 and printed circuit 74 attached tosurface 71 in a similar manner; and the circuitry and wiring associatedwith the gauges is parallel to that of the cone load cell, the wiringfrom the printed circuit extending through apertures 76 to the hollowinterior of the load cell.

In operation, the penetrometer probe of FIGS. 1 to 4 is pushed into theground by a drilling rig or similar apparatus, usually hydraulicallyoperated. The rig operates to push the pushing rods, the lowermost ofwhich is screwed to the adaptor 12, at a carefully controlled constantrate of, preferably, 2 cm/sec. The rods are screwthreaded, internally atone end, and externally at the other, so that they may be screwedtogether to form a continuous rod structure which can extend to adistance of 45 m. It is desirable to use a rig which has a stableplatform from which to operate. Stability and careful control of probevelocity significantly increase the accuracy of results obtained in anysingle test.

FIG. 5 is a block diagram of the probe strain gauge and associatedcircuitry and electrical equipment, which is advantageously all batteryoperated with rechargeable batteries. The rectangle 110 depicted inbroken lines contains the electrical equipment included within one loadcell. The other load cell equipment is identical, and accordingly neednot be shown.

Equipment 110 includes a balancing bridge 112 which incorporates thestrain gauges 114 of a load cell, a balancing potentiometer 113, and anadditional potentiometer 111, which acts as a vernier adjustment to thepotentiometer 113. This enables accurate balance to be undertaken withinthe probe, such that minimum balancing needs to be carried out at thedisplay equipment end. Voltage regulators 116 are provided in thecircuit.

The wiring from equipment 110 travels through cable 118, to amplifier120, which is associated with a balancing net 124 and a high qualityvoltage regulator 122. The signal from equipment 110 is fed to a digitaldisplay 126, preferably a liquid crystal display, which provides acontinuous calibrated quantitative visual readout of the cone resistanceor the side friction. In practice, due to the difference in order of thequantities involved, cone resistance R is displayed in MPa, and sidefriction R in KPa. The signal to the display can also be fed to amicroprocessor 128, for further immediate soil analysis. The signal canalso be fed to a chart recorder 130, for a continuous `hard copy` recordof the measured quantities. In practice, graphs are produced of coneresistance (R), side friction (F), and friction ratio (F/R), each as afunction of probe depth.

Block 132 contains circuitry which also receives the signal from theparticular load cell; the circuitry operates to stop the rod pushingoperation once either of the measured quantities exceeds a predeterminedvalue, thus preventing damage to the probe.

An example is shown within block 132, where a potentiometer 133 is usedto set a predetermined maximum signal level; a relay 134 is actuatedwhen that level is reached. The relay, when actuated, acts to switch offa pushing machine, or as shown, closes a valve in the hydraulic system,part of which is designated by reference numeral 136.

The claims form part of the disclosure of this specification.

I claim:
 1. A quasi-static electrical friction-sleeve cone penetrometercomprising; a generally cylindrical probe including a main body, a firstload cell integral with said body and having a longitudinal axis, anindependent second load cell connected to said body, said first loadcell and said second load cell being axially spaced along saidlongitudinal axis, a cone connected to said first load cell, and afriction sleeve connected to said second load cell, said friction sleevehaving an inwardly extending portion intermediate of the length thereofforming a sliding support for said sleeve on said main body between saidfirst load cell and said second load cell, first electrical meansassociated with said first load cell for conversion of first load celldistortion as a result of cone resistance into electrical signals, andsecond electrical means associated with said second load cell forconversion of second load cell distortion as a result of side frictionon said friction sleeve into electrical signals to provide a continuousand separate measurement of each quantity.
 2. A penetrometer as setforth in claim 1, wherein each of said first electrical means and saidsecond electrical means includes a series of strain gauges bonded to theexternal surface of the load cell, said gauges being electricallyconnected to a flexible printed circuit bonded to said surface, saidprinted circuit having two parallel rows of connectors, each row beinginterrupted, the interrupted portions of each row being staggered.
 3. Apenetrometer as set forth in claim 1, further including an amplifier foramplifying said electrical signals, a digital display for quantitativelydisplaying the quantities measured by said strain gauges, and chartrecorders for recording cone resistance R, side friction F and frictionratio F/R.
 4. A penetrometer as set forth in claim 3, further includingcut-off means, operative when one of said quantities reaches apredetermined value, to prevent further advancement of the penetrometerprobe.
 5. A penetrometer as set forth in claim 1 wherein the probe isadvanced at a controlled velocity of 2 cm/sec.
 6. A quasi-staticelectrical friction sleeve cone penetrometer comprising a generallycylindrical probe including a main body, a first load cell integral withsaid body, a second load cell connected to said body, a cone connectedto said first load cell, and a friction sleeve connected to said secondload cell, first electrical means associated with said first load cellfor conversion of first load cell distortion as a result of coneresistance into electrical signals, and second electrical meansassociated with said second load cell for conversion of second load celldistortion as a result of side friction on said friction sleeve intoelectrical signals to provide a continuous and separate measurement ofeach quantity; and each of said first electrical means and said secondelectrical means including a series of strain gauges bonded to theexternal surface of the corresponding load cell, each of said gaugesbeing electrically connected to a flexible printed circuit bonded tosaid surface, each of said printed circuits having two parallel rows ofconnectors, each row being interrupted, the interrupted portions of eachrow being staggered.
 7. A penetrometer as set forth in claim 6, whereineach of said first electrical means and said second electrical meansfurther includes a bridge network incorporating said strain gauges andsaid printed circuit, the network including a balancing potentiometerand a vernier potentiometer on the movable arm of the balancingpotentiometer.
 8. A penetrometer as set forth in claim 6 furtherincluding an amplifier for amplifying the signals from the straingauges, a digital display for quantitatively displaying the quantitymeasured by said strain gauges, and chart recorders for recording coneresistance R, side friction F and friction ratio F/R.
 9. A quasi-staticelectrical friction sleeve cone penetrometer comprising a generallycylindrical probe including a main body, a first load cell integral withsaid body, a second load cell connected to said body, a cone connectedto said second load cell, first electrical means associated with saidfirst load cell for conversion of first load cell distortion as a resultof cone resistance into electrical signals, and second electrical meansassociated with said second load cell for conversion of second load celldistortion as a result of side friction on said friction sleeve intoelectrical signals to provide a continuous and separate measurement ofeach quantity; annular sleeves provided on said main body, said secondload cell or said friction sleeve, and said cone, for supporting saidfriction sleeve at three points against lateral forces; and each of saidfirst electrical means and said second electrical means including aseries of strain gauges bonded to the external surface of thecorresponding load cell, each of said gauges being electricallyconnected to a flexible printed circuit bonded to said surface, each ofsaid printed circuits having two parallel rows of connectors, each rowbeing interrupted, the interrupted portions of each row being staggered.10. A penetrometer as set forth in claim 9 further including anamplifier for amplifying the signals from the strain gauges, a digitaldisplay for quantitatively displaying the quantity measured by saidstrain gauges, and chart recorders for recording cone resistance R, sidefriction F and friction ratio F/R.
 11. A penetrometer as set forth inclaim 9, wherein each of said first electrical means and said secondelectrical means further includes a bridge network incorporating saidstrain gauges and said printed circuit, the network including abalancing potentiometer and a vernier potentiometer on the movable armof the balancing potentiometer.